1. Field of the Invention
The present invention relates to communication systems, and more particularly to methods and apparatuses for minimizing signal interference.
2. Description of the Related Art
FIG. 1 illustrates a typical satellite television system of the related art.
FIG. 1 shows a communications system, specifically a television broadcasting system 100, which transmits and receives audio, video, and data signals via satellite. Although the present invention is described in the context of a satellite-based television broadcasting system, the techniques described herein are equally applicable to other methods of program content delivery, such as terrestrial over-the-air systems, cable-based systems, and the Internet. Further, while the present invention will be described primarily with respect to television content (i.e. audio and video content), the present invention can be practiced with a wide variety of program content material, including video content, audio content, audio and video related content (e.g., television viewer channels), or data content (e.g., computer data).
Television broadcasting system 100 includes transmission station 102, uplink dish 104, at least one satellite 106, and receiver stations 108A-108C (collectively referred to as receiver stations 108). Transmission station 102 includes a plurality of inputs 110 for receiving various signals, such as analog television signals, digital television signals, video tape signals, original programming signals and computer generated signals containing HTML content. Additionally, inputs 110 receive signals from digital video servers having hard discs or other digital storage media. Transmission station 102 also includes a plurality of timing inputs 112, which provide electronic schedule information about the timing and content of various television channels, such as that found in television schedules contained in newspapers and television guides.
Program guide data may also be manually entered at the site of transmission station 102. The program guide data consists of a plurality of “objects”. The program guide data objects include data for constructing an electronic program guide that is ultimately displayed on a user's television monitor.
Transmission station 102 receives and processes the various input signals received on inputs 110 and timing inputs 112, converts the received signals into a standard form, combines the standard signals into a single output data stream 114, and continuously sends output data stream 114 to uplink dish 104. Output data stream 114 is a digital data stream that is typically compressed using MPEG-2 encoding, although other compression schemes, such as MPEG-4 or other schemes, may be used.
The digital data in output data stream 114 are divided into a plurality of packets, with each such packet marked with a Service Channel Identification (SCID) number. The SCIDs can be used by a receiver in receiver station 108 to identify the packets that correspond to each television channel. Error correction data is also included in output data stream 114.
Output data stream 114 is typically a multiplexed signal that is modulated by transmission station 102 using standard frequency and polarization modulation techniques. Output data stream 114 preferably includes a plurality of frequency bands, typically sixteen frequency bands, with each frequency band being either left polarized or right polarized. Alternatively, vertical and horizontal polarizations may be used.
Uplink dish 104 continuously receives output data stream 114 from transmission station 102, amplifies the received signal and transmits signal 116 to at least one satellite 106. Although a single uplink dish 104 and three satellites 106 are shown in FIG. 1, multiple uplink dishes 104 and a larger number of satellites 106 are preferably used to provide additional bandwidth, and to help ensure continuous delivery of signals 114 to receiver stations 108.
Satellites 106 revolve in geosynchronous orbit about the earth. Satellites 106 each include a plurality of transponders that receive signals 116 transmitted by uplink dish 104, amplify the received signals 116, frequency shift the received signals 116 to different frequency bands, and then transmit the amplified, frequency shifted signals 118 back to desired geographic areas on the Earth, where receiver stations 108 are located or will be located at some time in the future. Receiver stations 108 then receive and process the signals 118 transmitted by satellites 106.
Each satellite 106 typically broadcasts signals 118 in thirty-two (32) different frequencies, which are licensed to various users for broadcasting of programming, which can be audio, video, or data signals, or any combination. These signals are typically located in the Ku-band of frequencies, i.e., 11-18 GHz, but can be broadcast in the Ka-band of frequencies, i.e., 18-40 GHz, more typically in the 20-30 GHz range, or other frequency bands.
FIG. 2 is a block diagram of one of receiver stations 108, which receives and decodes audio, video and data signals. Typically, receiver station 108 is a “set top box,” also known as an Integrated Receiver Decoder (IRD), which is usually resident in a home or multi-dwelling unit, for reception of satellite broadcasted television signals 118. Receiver station 108 may also be a Personal Video Recorder (PVR) which can record signals for playback at a later date.
Receiver dish 200 can be an Outdoor Unit (ODU), which is usually a smaller dish antenna mounted on a home or multi-dwelling unit. However, receiver dish 200 can also be a larger ground-mounted antenna dish if desired.
Receiver dish 200 typically uses a reflector dish and feedhorn assembly to receive and direct downlink signals 118 to receiver station 108 via a wire or coaxial cable. Each receiver station has a dedicated cable that allows receiver dish 200, via a multiswitch, to selectively direct downlink signals 118 to receiver station 108, and allows receiver station 108 to determine which of the signals 118 is desired.
Receiver station 108 typically includes receiver dish 200, alternate content source 202, receiver 204, monitor 206, recording device 208, remote control 210 and access card 212. Receiver 204 includes tuner 214/demodulator/Forward Error Correction (FEC) decoder 216, digital-to-analog (D/A) converter 218, CPU 220, clock 222, memory 224, logic circuit 226, interface 228, infrared (IR) receiver 230 and access card interface 232. Receiver dish 200 receives signals 118 sent by satellites 106, amplifies the signals 118 and passes the signals 118 on to tuner 214. Tuner 214 and demodulator/FEC decoder 216 operate under control of CPU 220.
The CPU 220 operates under control of an operating system stored in the memory 224 or within an auxiliary memory within the CPU 220. The functions performed by CPU 220 are controlled by one or more control programs or applications stored in memory 224. Operating system and applications are comprised of instructions which, when read and executed by the CPU 220, cause the receiver 204 to perform the functions and steps necessary to implement and/or use the present invention, typically, by accessing and manipulating data stored in the memory 224. Instructions implementing such applications are tangibly embodied in a computer-readable medium, such as the memory 224 or the access card 212. The CPU 220 may also communicate with other devices through interface 228 or the receiver dish 200 to accept commands or instructions to be stored in the memory 224, thereby making a computer program product or article of manufacture according to the invention. As such, the terms “article of manufacture,” “program storage device” and “computer program product” as used herein are intended to encompass any application accessible by the CPU 220 from any computer readable device or media.
Memory 224 and access card 212 store a variety of parameters for receiver 204, such as a list of channels receiver 204 is authorized to process and generate displays for; the zip code and area code for the area in which receiver 204 is used; the model name or number of receiver 204; a serial number of receiver 204; a serial number of access card 212; the name, address and phone number of the owner of receiver 204; and the name of the manufacturer of receiver 204.
Access card 212 is removable from receiver 204 (as shown in FIG. 2). When inserted into receiver 204, access card 212 is coupled to access card interface 232, which communicates via interface 228 to a customer service center (not pictured). Access card 212 receives access authorization information from the customer service center based on a user's particular account information. In addition, access card 212 and the customer service center communicate regarding billing and ordering of services.
Clock 222 provides the current local time to CPU 220. Interface 228 is preferably coupled to a telephone jack 234 at the site of receiver station 108. Interface 228 allows receiver 204 to communicate with transmission station 102 as shown in FIG. 1 via telephone jack 234. Interface 228 may also be used to transfer data to and from a network, such as the Internet.
The signals sent from receiver dish 200 to tuner 214 are a plurality of modulated Radio Frequency (RF) signals. The desired RF signal is then downconverted to baseband by the tuner 214, which also generates in-phase and quadrature-phase (I and Q) signals. These two signals are then passed to the demodulator/FEC Application Specific Integrated Circuit (ASIC) 216. The demodulator 216 ASIC then demodulates the I and Q signals, and the FEC decoder correctly identifies each transmitted symbol. The received symbols for Quaternary Phase Shift Keying (QPSK) or 8PSK signals carry two or three data bits, respectively. The corrected symbols are translated into data bits, which in turn are assembled in to payload data bytes, and ultimately into data packets. The data packets may carry 130 data bytes or 188 bytes (187 data bytes and 1 sync byte).
In addition to the digital satellite signals received by receiver dish 200, other sources of television content are also preferably used. For example, alternate content source 202 provides additional television content to monitor 206. Alternate content source 202 is coupled to tuner 214. Alternate content source 202 can be an antenna for receiving off the air signals National Television Standards Committee (NTSC) signals, a cable for receiving American Television Standards Committee (ATSC) signals, or other content source. Although only one alternate content source 202 is shown, multiple sources can be used.
Initially, as data enters receiver 204, CPU 220 looks for initialization data which is referred to commonly in the industry as a boot object. A boot object identifies the SCIDs where all other program guide objects can be found. Boot objects are always transmitted with the same SCID, so CPU 220 knows that it must look for packets marked with that SCID. The information from the boot object is used by CPU 220 to identify packets of program guide data and route them to memory 224.
Remote control 210 emits Infrared (IR) signals 236 that are received by infrared receiver 230 in receiver 204. Other types of data entry devices may alternatively be used, by way of example and not limitation, such as an ultra-high frequency (UHF) remote control, a keypad on receiver 204, a remote keyboard and a remote mouse. When a user requests the display of a program guide by pressing the “guide” button on remote control 210, a guide request signal is received by IR receiver 230 and transmitted to logic circuit 226. Logic circuit 226 informs CPU 220 of the guide request. In response to the guide request, CPU 220 causes memory 224 to transfer a program guide digital image to D/A converter 218. D/A converter 218, converts the program guide digital image into a standard analog television signal, which is then transmitted to monitor 206. Monitor 206 then displays the TV video and audio signals. Monitor 206 may alternatively be a digital television, in which case no digital to analog conversion in receiver 204 is necessary.
Users interact with the electronic program guide using remote control 210. Examples of user interactions include selecting a particular channel or requesting additional guide information. When a user selects a channel using remote control 210, IR receiver 230 relays the user's selection to logic circuit 226, which then passes the selection on to memory 224 where it is accessed by CPU 220. CPU 220 performs an MPEG2 decoding step on received audio, video, and other packets from FEC decoder 216 and outputs the audio and video signals for the selected channel to D/A converter 218. D/A converter 218 converts the digital signals to analog signals, and outputs the analog signals to monitor 206.
Such communications systems 100, here by example which is shown a television broadcast system 100, have embraced the demand for high quality transmissions made possible by digital technology. As the packets and other data are transmitted from uplink dish 104 to receiver 108, the symbols and bits in packets intended for other receiver stations 108 are typically transmitted down from satellite 106 to receiver 108 on the same frequency, because the transmit frequency is controlled by the limitations of satellites 108, and the transmit frequencies that are available are controlled by government permission for transmission at specific frequencies within the frequency spectrum.
Further, the data frames are coded in such a manner that they can interfere with each other, and receiver 108 cannot tell which packets of data that receiver 108 is supposed to decode and present on monitor 206. Such interference is called “co-channel” interference, where one channel of data interferes with the reception and demodulation of another channel of data. In practical applications, the co-channel interference may also stem from transmission of other system operators, a satellite 106 operating in an adjacent orbital slot, or other spot transmission beams in a spot beam satellite broadcasting system 100.
As communications systems 100 transmits more data, i.e., more channels of programming on a satellite broadcast system that are viewable on monitor 206, the interference between data transmission will increase, and, as such, the quality of the signal reception will be poorer.
To make optimal use of the available spectrum and to deliver a high number of different channels of programming, RF transmissions with the same frequencies may be directed to different geographic areas. However in areas bordering the different service areas, it is possible that a receiving station may detect a wanted transmission, but also other co-frequency transmissions. The unwanted transmissions are interference and may severely degrade the overall performance of the wanted channel receiver.
Traditionally, the negative effects of co-channel interference have been minimized by redesigning the frequency assignments assigned to the various transponders or satellites 106. But this will not alleviate the problem beyond a certain point.
It can be seen, then, that there is a need in the art to minimize the interference in a broadcasting system.